Obesity is an epidemic with enormous health, economic, and social burdens. Several neuropeptides were found to be involved in regulating energy homeostasis. These neuropeptides act on their cell surface receptors to exert their physiological effects. The melanocortin-3 receptor (MC3R) is increasingly being recognized as an important regulator of energy homeostasis, especially for fat deposit. It is also involved in regulating cardiovascular function and inflammation. The applicant's long-term goals are to understand the structure-function relationships of the human MC3R at the molecular level. The research proposed in this application is aimed at understanding the molecular mechanisms of MC3R coupling to G proteins. The following two specific aims will be pursued: (1) To characterize the intracellular surface of the MC3R that are involved in G protein coupling; (2) To map some of the contact sites between the MC3R and the stimulatory G protein. Functional studies of alanine scanning mutants will be performed to achieve the objectives of Aim 1. For Aim 2, co-transfection in cells lacking the stimulatory G protein will be used to probe the contact sites between the MC3R and the stimulatory G protein. The results of these studies will advance our understanding of the mechanisms of the MC3R coupling to the stimulatory G protein. They should also be of great interest for the GPCR-G protein field in general, where detailed mapping of the contact sites between GPCRs and their cognate G proteins has rarely been done.Obesity is an epidemic in the US with enormous health, economic and social costs. Current pharmacotherapies are far from ideal in terms of both efficacy and side effect. Our long-term goal is to exploit the melanocortin system as a novel approach for obesity treatment. [unreadable] [unreadable] [unreadable]